Recently, a secondary battery has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle or large-sized devices, such as vehicles, use a middle or large-sized battery module having a plurality of battery cells electrically connected to each other because high power and capacity are necessary for the middle or large-sized devices.
Preferably, the middle or large-sized battery module is manufactured so as to have as small a size and weight as possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the middle or large-sized battery module. In particular, much interest is currently focused on such a pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the pouch-shaped battery is lightweight, the manufacturing cost of the pouch-shaped battery is low, and it is easy to modify the shape of the pouch-shaped battery.
In order for the middle or large-sized battery module to provide power and capacity required by a specific apparatus or device, it is necessary for the middle or large-sized battery module to be configured to have a structure in which a plurality of battery cells is electrically connected in series to each other, and the battery cells are stable against an external force.
Also, the battery cells constituting the middle or large-sized battery module are secondary batteries which can be charged and discharged. Consequently, a large amount of heat is generated from the high power, high capacity secondary batteries during the charge and discharge of the batteries. If the heat, generated from the unit cells during the charge and discharge of the unit cells, is not effectively removed, the heat accumulates in the respective unit cells with the result that the deterioration of the unit cells is accelerated. According to circumstances, the unit cells may catch fire or explode. For this reason, a cooling system is needed in a battery pack for vehicles, which is a high power, high capacity battery, to cool battery cells mounted in the battery pack.
In a middle or large-sized battery pack including a plurality of battery cells, on the other hand, the deterioration in performance of some battery cells leads to the deterioration in performance of the entire battery pack. One of the main factors causing the non-uniformity in performance is the non-uniformity of cooling between the battery cells. For this reason, it is necessary to provide a structure to secure the uniformity of cooling during the flow of a coolant.
Some conventional middle or large-sized battery packs use a battery pack case configured to have a structure in which a coolant inlet port and a coolant outlet port are located at the upper part and the lower part of the battery pack case such that the coolant inlet port and the coolant outlet port are directed in opposite directions, and the top and bottom of a flow space extending from the coolant inlet port to the battery module are parallel to each other. In this structure, however, a relatively high coolant flux is introduced into flow channels defined between the battery cells adjacent to the coolant outlet port, whereas a relatively low coolant flux is introduced into flow channels defined between the battery cells adjacent to the coolant inlet port with the result that it is difficult to achieve uniform cooling of the battery cells.
In connection with this matter, Korean Patent Application Publication No. 2006-0037600, No. 2006-0037601, and No. 2006-0037627 disclose a middle or large-sized battery pack configured to have a structure in which an air guide plane is inclined downward to a side of a battery pack case opposite to battery cells so that the air guide plane becomes closer to the battery cells with the increase in distance between the air guide plane and a coolant inlet port. Specifically, the air guide plane is inclined at a predetermined angle, for example an angle of 15 to 45 degrees, to the side of the battery pack case opposite to the battery cells, thereby restraining the occurrence of a phenomenon in which a coolant is excessively introduced into flow channels defined between the battery cells adjacent to the coolant outlet port.
However, the inventors of the present application have found that the temperature deviation between the battery cells is high even in the above-mentioned structure with the result that it is not possible to achieve temperature uniformity of a desired level.
In a battery pack for vehicles, on the other hand, energy density is increased with the increase of required battery capacity, and therefore, the total size of the battery pack is increased. However, a space for the batter pack is not sufficient with the result that a space occupied by flow channels in the battery pack is gradually decreased.
Also, in a conventional battery pack, the width of a cooling flow channel is identical to the width of a battery module or a battery cell. For this reason, the height of the cooling flow channel is decreased so as to increase energy density within the limited size of the battery pack. If the height of the cooling flow channel is decreased, however, pressure applied to the cooling flow channel is increased with the result that flow deviation is increased.
Consequently, there is a high necessity for a technology to fundamentally solve the above-mentioned problems.